Two critical factors in the flight of any aircraft are the weight and balance of that, aircraft. An airplane manufacturer must publish the maximum gross weight of that airplane. This is to insure that at take-off speed, the wings are generating sufficient lift to lift the weight of the airplane. An equally important factor to consider is whether the airplane is in proper balance (center of gravity) or within acceptable limits, as can be compensated for by trim adjustments.
The weight of an aircraft is supported on a plurality of collapsible landing gear struts. These landing gear struts contain pressurized hydraulic fluid and nitrogen gas. The pressure within each landing gear strut is related to the amount of weight that landing gear strut is supporting. Aircraft landing gear struts incorporate the shock absorbing technique of forcing hydraulic fluid through an orifice within the strut. The nitrogen gas is an additional cushioning agent. Multiple O-ring seals within the landing gear strut are used to retain the hydraulic fluid and compressed nitrogen gas contained within each landing gear strut. The retention of the compressed nitrogen gas and hydraulic fluid by the O-ring seals is due to the extreme amount of friction these seals maintain as they move up and down the cylinder walls of the landing gear strut. This friction (defined in the aircraft strut industry as "stiction"), while it may improve the shock absorbing quality of the landing gear strut, distorts internal landing gear strut pressures, as those pressures relate to the amount of weight the landing gear strut is supporting. Compensations are needed to correct for distorted pressure readings caused by the stiction within these landing gear struts in order to accurately determine the aircraft weight.
Previous systems to determine gross weight and center of gravity are well known and well documented. Reference may be made to U.S. Pat. Nos. 3,513,300 Elfenbein, 3,581,836 Segerdahl, 5,521,827 Lindberg et al, and this inventor, 5,214,586 and 5,548,517 Nance.
U.S. Pat. No. 3,513,300 Elfenbein, identified the relationship between aircraft weight and the pressure within the landing gear struts. Elfenbein pioneered the art of measuring landing gear strut pressure and relating it to the amount of weight supported. The Elfenbein prior art does not compensate for landing gear strut pressure distortions caused by strut stiction.
U.S. Pat. No. 3,581,836 Segerdahl, identified friction as a factor causing errors in the relationship between the pressure within the landing gear struts and the aircraft weight. The Segerdahl prior art incorporates the practice of injecting and withdrawing fluid from the landing gear struts. Segerdahl teaches the practice of measuring the pressure of hydraulic fluid within the hydraulic line that is used to inject and withdraw hydraulic fluid into and from the landing gear strut. This practice measures pressure which is not solely related to weight supported and landing gear strut friction, but also measures the higher pressure of the hydraulic fluid injection mechanism and the lower pressure of the hydraulic fluid withdrawal mechanism. This false higher or lower pressure, which is assumed to be landing gear strut pressure, and which is used in the weight calculations, is distorted by the pressure differential between the pressure within the body of the landing gear strut and that of the higher or lower pressure of the hydraulic fluid injection mechanism. Erroneously high pressures are measured as hydraulic fluid is injected into the strut, as well as erroneously low pressures are measured as hydraulic fluid is withdrawn from the strut. There must be a substantially higher pressure in the hydraulic fluid injection mechanism, or the landing gear strut would not extend and a substantially lower pressure in the hydraulic fluid withdrawal mechanism, or the landing gear strut would not collapse. Segerdahl's prior art also ignores pressure fluctuations in the landing gear's nitrogen gas caused by the compression of that nitrogen gas. As hydraulic fluid is injected into a landing gear strut the nitrogen gas is compressed. The compression of the nitrogen gas generates heat. As the temperature of the compressed nitrogen gas increases, the pressure of the nitrogen gas, as well as the hydraulic fluid it is in direct contact with, will increase to a pressure higher than that pressure directly related to the landing gear strut friction and weight the strut is supporting. These sources of error are not recognized by the Segerdahl prior art.
U.S. Pat. No. 5,521,827 Lindberg et al, continues the Segerdahl and Nance (to be described below) prior art on the identification of friction as a factor causing errors in the direct relationship between the pressure within the landing gear struts and the aircraft weight. Lindberg teaches the practice of multiple hydraulic fluid injections raising each landing gear strut to near full extension and multiple hydraulic fluid withdrawals lowering each landing gear strut to near full collapse. Wile these extreme up and down movements, raising and lowering the aircraft as much as 2-3 feet, may offer some relief to the potential errors in the prior art taught by Segerdahl, such extreme aircraft movement is incompatible with today's aircraft loading procedures which utilizes a floating passenger "jet-bridge" adjacent to the aircraft door and baggage loading conveyor belts which extend directly into each of the aircraft's cargo compartments. Extreme aircraft movement could cause severe damage to the aircraft or injuries to passengers if the Lindberg practice were to be used during the aircraft loading process.
This invention relates to improvements to the above mentioned prior art as well as the prior art of this inventor (Nance) U.S. Pat. Nos. 5,214,586 and 5,548,517. The Nance technology, among other things, measures the pressure distortions caused by strut seal friction, then stores that information for future reference in the event the hydraulic fluid injection and withdrawal mechanism is not functioning. This technology incorporates the storage of defined pressure limits to be used in the determination of hard landings by the aircraft. This technology also measures strut fluid temperature and adjusts for pressure distortions caused by changes in temperature.